With the rapid development of an IMS (IP Multimedia Subsystem) in the field of telecommunication, an IMS network is becoming widely applied. However, the IMS network will not be fully deployed in a short period of time. Moreover, it is impossible to provide all CS (Circuit Switch) network users with new IMS terminals instead in a short period of time. Therefore, conventionally a CS network still has to coexist with the IMS network. As a result, simultaneous operation of CS and IMS service platforms increases the cost of constructing new services and the operating cost.
To meet a requirement of coexistence of the CS network and the IMS network at present, a CS network service logic function may be transferred to the IMS network to implement a united service platform, so as to reduce the cost of constructing and operating new services. Such technology is referred to as an ICS (IMS Centralized Service). The ICS may also be regarded as a new IMS session, in which the IMS session is supported to use a CS bearer (Bearer), which is alternatively referred to as a CS media, to transmit media streams, and multiple IMS sessions may share one CS bearer. The CS bearer is established through a CS call, where the CS call is converted into an IMS call by an MSC server (Mobile Switch Center server) or an MGCF (Media Gateway Control Function). An SCC AS (Service Centralization and Continuity Application Server) that supports the ICS is an IMS application based on a home network. A UE (User equipment) that supports the ICS establishes a service control channel with the SCC AS to transmit IMS service control signaling (such as call initiation, transfer and release), where the service control channel is based on a Gm interface or an I1 interface.
The terms “call” and “session” are not distinguished in this document, and are able to be replaced with each other.
CAT (Customized Alerting Tones), generally known as color ring back tones, are a service triggered in a phase of a session setup, and are capable of presenting multimedia information (such as one of or a combination of a speech, a video and a text) to a calling party to replace a conventionally used monotonous ringback tone (for prompting a ring to a called terminal). The CAT may also be regarded as a new ringback tone, which allows a user or an operator to customize media content of the ringback tone.
The CAT, which mainly transmits media streams before the called terminal sends an off-hook signal, belongs to an early session (or referred to as early media), and a regular session (that is, transmission of a normal call media streams) is performed after the off-hook signal. Another common type of early media is Announcement, for example, for prompting the user that “the network is busy, please dial again later”. The early media mainly have three service models: a gateway model, a forking model and an early-session model. The early media may still exist after the off-hook signal. For example, the CAT media may serve as, for example, background music or a background video, after a normal session starts.
As regards implementing the early media (for example, a CAT service) during ICS session setup, the inventor finds the following problems:
While initiating setup of a first ICS session, an ICS UE also initiates setup of a CS call, where the CS call is converted into an IMS call by the MSC server or the MGCF. Therefore, the MSC server or the MGCF supports the early media at this time. However, when the ICS UE initiates a second ICS session, an existing CS bearer is reused, that is, a setup process of a CS call does not exist, and therefore, the MSC server or the MGCF does not support the early media.
As for a case of forking of the called party occurring in the setup of the ICS session, the following problems exist:
Similar to the early media, the forking is also a mechanism of the call setup process. Therefore, when the ICS UE initiates the second ICS session, the MSC server or the MGCF cannot support the forking.